1. Field of the Invention
The present invention relates to an alignment measurement system which accurately and precisely measures the location of alignment marks on a manufactured substrate.
2. Description of the Background Art
Phase-grating interference, as a principle, has been used for reticle and wafer position detection for stepper alignment systems. See, for example: S. Austin, et al., Applied Physics Letters, Vol. 31, p. 426 (1977); U.S. Pat. No. 4,251,160 (1978); U.S. Pat. No. 4,828,392 (1989); U.S. Pat. No. 5,144,363; and U.S. Pat. No. 6,844,914 (2005).